This invention relates to thick film hybrid manufacturing techniques, and more particularly to apparatus for contact printing paste solder onto both component and lead pads of a leaded hybrid substrate in a single solder screening operation.
The conventional method of manufacturing thick film hybrid circuits requires two different solder application steps. In the first step, paste solder is printed onto only component pads on a substrate. After component terminals are located in paste solder, the substrate is reflow soldered to electrically connect the components into electrical circuit patterns on it. A lead frame is then mechanically attached to the substrate by pressing tines of leads over one edge of the substrate so that some tines are on lead pads. In the second step, the leads and one edge of the substrate are dipped into molten solder. This manufacturing method requires considerable operator time. Although a dip solder operation can provide a relatively uniform and thin solder coating of lead tines and associated pads, extreme care must be exercised to ensure that a substrate is dipped deep enough into the molten solder without loosening reflow soldered components adjacent the lead pads such that they move or fall into the solder bath. In order to reduce the number of steps such as storage, cleaning, soldering and inspection that are required in fabricating hybrid circuits, an operator will sometimes use a syringe to individually apply paste solder to both component pads and lead pads having tines thereon prior to a single solder reflow operation. Since this method of dispensing paste solder is operator dependent, it requires considerable operator time. Also, it does not result in uniform amounts of solder being consistently applied to the pads. This causes non-uniform solder junctions at lead contacts that do not have consistent levels of shear strength. Non-contact printing of paste solder using a constant thickness metal stencil having apertures therein in the same pattern as pads on substrates has also been tried. This non-contact printing technique is similar to applying paste solder with a syringe and has also been found unsatisfactory.
An object of this invention is the provision of improved apparatus for more consistently applying prescribed amounts of paste solder to lead and component pads on a leaded substrate during a single printing operation so that lead tines and component terminals may be soldered to associated pads in a single reflow solder operation.
Another object is the provision of an improved stencil for use in screening paste solder onto lead and component pads of a leaded substrate in a single printing operation.